Well, that is unfortunate... sometimes it happens, sometimes it doesn't.

Okay. You know, just saying "Sometimes it happens" already tells you that something happens sometimes and other times it doesn't. But, sometimes, I need to make very clear that this "something" doesn't happen always. That's why I say

sometimes it happens, sometimes it doesn't

But is there a way to shorten this phrase? To make it simpler, but still making very clear the fact that this "something" may not happen? Repeating "sometimes" is not very much of my liking...